This invention relates to apparatus for sampling the emissions in a gas flow and, more particularly, to apparatus adapted for isokinetic sampling of a gas flow.
Sampling apparatus heretofore available for measuring emissions, i.e., particulate matter, under isokinetic flow conditions has required considerable manual adjustment before a single reading in a stack or other conduit could be achieved. An apparatus of this type is shown in Boubel, patent application Ser. No. 385,310, filed Aug. 3, 1973, now U.S. Pat. No. 3,841,145. This sampler comprises a generally cylindrical tube and includes an inlet nozzle disposed at the forward end of the tube and adapted for insertion into a stack or like conduit through which a gas is flowing. A Pitot tube is attached to the nozzle and is adapted for insertion into the stack with the nozzle for measuring the velocity of gas flowing through the stack. A filter is disposed in the tube downstream of the nozzle for collecting particulate matter. The filter is disposed in a housing which comprises upstream and downstream sections removably connected together to facilitate rinsing of the sampler upstream of the filter as is necessary fully to account for particulate matter removed from the flow.
The sampler further comprises a suction blower which is connected to the downstream end of the tube. A flow measuring orifice is disposed in the tube between the filter and the blower and a control valve is provided for regulating the flow therethrough.
In using a sampler of this type to measure emissions in a stack or similar conduit, the temperature of the gas flowing through the stack is first measured and the inlet nozzle with the Pitot tube attached is inserted into the stack to measure the velocity pressure. Appropriate calibration curves are used to convert the reading on a velocity pressure gauge into a gas velocity or volume rate of flow through the stack.
The suction blower is then started and flow is drawn through the tube to determine the average sample temperature therein. Once the temperature is determined, other calibration curves are used to calculate the reading that should be obtained on a pressure gauge which measures the pressure drop across the orifice to determine the flow through the sampler that will achieve isokinetic conditions. The flow is then regulated by means of the control valve such that the actual reading on the pressure gauge corresponds to the calculated value.
When the flow through the sampler has been thus "matched" to the velocity of the gas flow in the stack, a particulate sampling can be taken on the filter. The sampling period is chosen such that it is long enough to obtain a sufficient sample for an accurate weight determination. Assuming the gas flow in the stack to remain at a constant temperature and velocity, the value obtained will be accurate with no adjustment of the control valve required.
Governmental authorities, however, often require that particulate emissions be measured at different locations across a stack. These locations often have different temperature and velocity readings. Such requires continual adjustment of the apparatus by the operator in order to maintain isokinetic flow. Also, the flow at a given point in a stack often varies and is not constant over the sampling period, such that inaccuracies in a reading inevitably occur.
Governmental authorities also require that the particulate emssions for a given reading be related to the particular volume of gas flow in which such emissions occur. Obtaining such a total volume requires an integration of the stack velocity over the period of time consumed during the test. This represents a further source of error, as will be appreciated.
Finally, manual adjustment of a sampler to achieve isokinetic conditions unavoidably incorporates operator error in measuring the several quantities required to make the determination and in adjusting the flow as required.
It is thus the principal object of the present invention to provide an isokinetic sampling apparatus of the type described that will be able automatically to compensate for varying gas temperatures and velocities in a given stack or other conduit in which particulate emissions are to be measured.
It is a further object of the present invention to provide such a sampler that will be able to measure the total volume of flow against which the particulate emissions obtained can be related.
It is a still further object of the present invention to provide a sampler of the type described that will minimize error and achieve increased accuracy in measuring particulate emissions.